If it was ok before the problem then it could be a high idle switch(as it's called).
To check drive at say 25mph and let it coast, high idle swicth if working will do just that say high idle.
If you arer saying it is a conversion one that does not have a switch but the original ecu used one then yes it needs one, easy to fit on the prop with magnets and pickup, they merely sense movement and output a digital signal ie on off, as magnets pass the sensor, and so cause disturbance to magnetic field created.
tabetha